A secondary battery is used for low-current household device applications such as notebook PC and cellular phone, or as a storage battery of a hybrid car, an electric car or the like. In these applications, as the secondary battery, a lithium ion secondary battery that is a nonaqueous electrolyte secondary battery is frequently used, because it has a high energy density.
In general, an electrode having a current collector such as metal foil and an electrode mixture layer provided on the current collector is used as the electrode of a nonaqueous electrolyte secondary battery, and an electrode active material and a conduction aid are held by a binder in the electrode mixture layer. Such an electrode is produced by kneading an electrode active material, a conduction aid, a binder and a liquid medium to prepare a slurry, coating the slurry on one surface or both surfaces of a current collector by means of a transfer roller or the like, removing the liquid medium by drying to form an electrode mixture layer, and thereafter, if desired, compression-molding the assembly by means of a roll press machine or the like. As the liquid medium, a medium capable of dispersing an electrode active material and a conduction aid and dissolving a binder is used.
Conventionally, polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) has been used as the binder for an electrode of a nonaqueous electrolyte secondary battery. In particular, PVDF has the advantages of, for example, exhibiting good rheology characteristics (thixotropy) when formed into a slurry and being electrochemically stable in a positive or negative electrode, and therefore is used for general purposes.
However, PVDF or SBR has a problem of low binding force. Therefore, when PVDF or SBR is used as the binder, it has been difficult to increase the capacity of the nonaqueous electrolyte secondary battery or enhance the battery performance such as rate characteristics and cycle characteristics. For example, an increase in the amount of a conduction aid is effective in enhancing the rate characteristics which are affected by the ease of electron transfer. In order to increase the amount of the conduction aid in a limited space inside a battery, the amount of the binder must be reduced, but when the binder amount is reduced, the adherence between the current collector and the electrode mixture layer or the adherence between electrode active materials is reduced and the electrode mixture layer is separated from the current collector due to repeated charging/discharging or the electrode active material drops off from the electrode mixture layer, leading to reduction in the battery performance.
To solve such problems, a method of specifying various parameters so as to achieve adherence or the like to the current collector has been proposed. For example, Patent Document 1 has proposed a method where in a binder composition containing a polymer having a THF gel content of 5% or less and an organic solvent using N-methylpyrrolidone (NMP) as the main solvent, the second virial coefficient measured by a static light scattering method is specified as being not more than a specific value and the radius of gyration of the polymer is specified as falling in a specific range. Also, Patent Document 2 has proposed a method where two kinds of dispersion liquids each obtained by dispersing a polymer having a primary particle mode diameter in a specific range are mixed in specific blending amounts.
Further, as the method for improving such defects, there has been proposed a method where an acidic group such as carboxyl group and phosphoric acid group is introduced into a polyacrylonitrile-based (hereinafter, may be referred to as “PAN-based”) resin having an electrochemical stability comparable to that of PVDF and the binding property of the binder with a current collector is thereby enhanced (see, Patent Documents 3, 4 and 5).